1. Field of the Invention
This invention relates to radiotherapy planning for Boron Neutron Capture Therapy (BNCT). More specifically, the present invention relates to a method for improving the simulation and display of BNCT isodoses superimposed upon the anatomical features of a patient that are to receive BNCT treatment.
2. Background Art
Application of neutrons for radiotherapy of cancer has been a subject of considerable clinical and research interest since the discovery of the neutron by Chadwick in 1932. Fast neutron radiotherapy was first used by Robert Stone in the Lawrence Berkeley Laboratory in 1938. This technology has evolved over the years to the point where it is now a very viable method for treating inoperable salivary gland tumors. On the basis of recent research data such technology also is emerging as a promising alternative for treatment for prostate cancer, some lung tumors, and certain other malignancies as well.
Neutron capture therapy (NCT), a somewhat different form of neutron-based therapy, was proposed in the mid 1930s and, despite some notable failures in early U.S. trials, has attracted a great deal of renewed research interest lately. This interest is due to significant improvements in radiobiological knowledge.
The basic physical processes involved in fast neutron therapy and neutron capture therapy differ in several respects. In fast neutron therapy, neutrons having relatively high energy (approximately 30-50 MeV) are generated by a suitable neutron source and used directly for irradiation of the treatment volume, just as is done with standard photon (x-ray) therapy. In neutron capture therapy, a neutron capture agent is injected into the patient and is selectively taken into the malignant tissue. The administration of a pharmaceutical containing the neutron capture agent is preferably direct administration into the bloodstream of the patient. At an appropriate time after administration of the neutron capture agent, the treatment volume (i.e., the anatomical structure to be treated) is exposed to a field of thermal neutrons produced by application of an external neutron beam. Because boron-10 is commonly used as the capture agent, the technology has come to be known as boron neutron capture therapy, or BNCT.
The thermal neutrons interact with the boron-10, which has a very high capture cross-section in the thermal energy range. Ideally, the boron-10 is present only in the malignant cells so that boron-neutron interactions will occur only in malignant cells. Each boron-neutron interaction produces an alpha particle and a lithium ion. These highly-energetic charged particles deposit their energy within a geometric volume that is comparable to the size of the malignant cell. Thus, boron-neutron interaction provides a high probability of cell inactivation by direct DNA damage.
Because boron is ideally taken up only in the malignant cells, the NCT process offers the possibility of highly selective destruction of malignant tissue while causing minimal damage to the normal tissue disposed adjacent to the tumor. When boron-10 is taken up in the malignant cells only, the separation between normal and malignant tissue occurs on a cellular-level basis--thereby providing considerable accuracy. In addition, the neutron sources used for NCT are, themselves, designed to produce a minimal level of damage to normal tissue which has not received the neutron capture agent.
When BNCT is administered as a primary therapy, an epithermal-neutron beam (neutrons having energies in the range of 1 eV to 10 keV) is used to produce the required thermal neutron flux at depth. This is because these somewhat higher-energy neutrons will penetrate deeper into the irradiation volume before thermalizing. Although the neutrons penetrate deeper, they are still not of sufficient energy to inflict unacceptable damage to intervening normal tissue.
A third form of neutron therapy is also a subject of current research interest. The third form of neutron therapy is basically a hybrid that combines the features of fast neutron therapy and NCT. In this type of radiotherapy, a neutron capture agent is introduced into a patient--preferably into the malignant tissue only. This treatment is prior to the administration of standard fast neutron therapy. Because a small fraction of the neutrons in fast neutron therapy will be thermalized in the irradiation volume, it is possible to obtain a small incremental absorbed dose from the neutron capture interactions that result. Thus, based on current radiobiological research, improved tumor control appears to be likely when using the augmentation concept.
One significant problem with the various neutron therapy systems is that they are usually located only at major research centers because they are physically complex, bulky, expensive to acquire and require high-level operating staffs to maintain. In general these systems are not well suited for wide-spread, practical, clinical deployment.
This disadvantage is compounded by the fact that in BNCT and other neutron therapy systems detailed planning calculations are necessary to optimize the treatment for each individual patient. Careful planning permit the delivery of the highest possible therapeutic radiation dose to the target tissue while maintaining the surrounding healthy tissue at or below tolerance. However, extensive planning can limit the number of patients which can be properly treated using neutron therapy equipment. Thus, in recent years significant efforts have been made to develop modern computational methods and software for use in BNCT treatment planning.
One such treatment planning system for BNCT has been developed by the New England Medical Center Hospitals. This system is described in U.S. Pat. No. 5,341,292 (Zamenhof), entitled Monte Carlo Based Treatment Planning for Neutron Capture Therapy. The Zamenhof system displays a patient image superimposed with isodose contour lines. To obtain a patient image superimposed with isodose contour lines, the system must process both a physical distribution and a biological distribution. Processing both the physical and biological distributions each time a user desires to view isodose contours on a patient image is inefficient and time consuming.
In addition, the Zamenhof system uses an undesirable method to eliminate unwanted isodose contours. Isodose contours appear everywhere that they are computed to appear--even in areas of the display that do not have a patient image. The isodose contours that appear outside the patient image are undesirable. To get rid of these contour lines, the Zamenhof method sets computed isodose values outside the patient image to zero. Zamenhof, Col. 2, lines 2-4. Setting the computed isodose values to zero in the air regions outside the patient image causes the isodose curves to have unrealistic dropouts near the margins of the patient image. These dropouts are sharp and cause a general distortion throughout the isodose curves.
Other treatment planning systems consider the isodose contours to consist of one component and present the contours on top of a medical image or graphical, visual representation of anatomical features. Although these systems increase the speed of treatment planning, they are not as accurate as desired.
Thus, there is a need for a method for treatment planning that provides for isodose contour line displays superimposed over a patient image, while eliminating the contour lines in areas not of concern to the user. Such a system should be easy to use and enable more efficient treatment planning.